This invention relates to spectrum analyzers such as are used in studying sea noise spectra, and more particularly to improvements in the AGC (automatic gain control) of sea noise spectrum analyzers, for example analyzers of the lofar (low frequency analyzer and recorder) type.
Signals received at underwater hydrophones generally include broadband noises due to wind and wave action, shipping flow noise, and seismic disturbances. Also present are discrete frequencies from marine propulsion machinery. A representative spectrum has a smooth background level with some peaks of discrete frequencies, the background level being substantially of the character of white noise.
When a spectrum analyzer output with widely varying spectral densities is presented on a limited dynamic display medium, an automatic gain control is needed. Traditionally, lofar analysis has been displayed on "Timefax" electrosensitive paper which has a very limited range of approximately 6 db.
Since a lofar spectrum may have a range of 40 db across the frequency band of interest, a stiff AGC is required to maintain the output within the 6 db range of the display medium. In a typical lofar spectrum analyzer with AGC, the AGC filter bandwidth is about 50 times the analyzing bandwidth, that is 10 Hz vs 0.2 Hz, and the AGC stiffness is about 1 db change of the output for a 15 db change of the input. Now, a strong signal within the AGC filter bandwidth causes a decrease in gain throughout the AGC filter bandwidth and hence a "whitening" of the recorded spectrum response on the electrosensitive paper about the strong signal to the width of the AGC filter. If weak signals are present at frequencies falling in the area of whitening, they will not be displayed. One prior art approach to overcoming the problem of whitening is found in U.S. Pat. No. 2,996,667 issued to R. R. Galbreath.
Recent improvements in filter techniques have made available narrow band pass filter devices such as monolithic crystal filters which are considerably more stable than filters obtainable before and which are more discriminating in their band pass characteristics.